The present invention relates generally to an improved inflatable article such as an inflatable ball, and particularly to an inflatable ball which can be inflated by smaller inflating pressure and which, when inflated, results in less deformation on its outer surface, thus reducing distortion of the words and patterns printed on the outer surface of the inflatable ball.
A typical example of a conventional inflatable ball is shown in FIG. 1. It comprises a plurality of side pieces 1 which are arranged and sealed side by side, a top piece 2, a bottom piece 3, and an inflation valve 4. Side pieces 1, top piece 2 and bottom piece 3 are all made of airtight, flexible thermoplastic materials so that they may be heat-sealed to one another. Usually the number of side pieces required for constituting an inflatable ball is eight or six. Since many working steps are necessary for forming such an inflatable ball, the production process is quite complicated and, consequently, the production cost is relatively high.
In view of the above-described drawbacks of the inflatable ball shown in FIG. 1, another type of inflatable ball as shown in FIG. 2 (in deflated state), and FIG. 3 (in an inflated state) has been proposed. As illustrated in these two figures, this inflatable ball comprises only three layers, namely an upper layer 21, an intermediate layer 22 and a lower layer 23 of thermoplastic sheets which are of similar elastic moduluses and are overlapped and heat-sealed along their circumferences. Several vent holes 25 are provided in intermediate layer 22, and an inflation valve 24 is mounted in upper layer 21 for inflating the inflatable ball. When a user (especially, a child) tries to inflate such an infatable ball by mouth, he will usually find it very difficult to inflate the inflatable ball into a really spherical shape due to the resistance of the upper layer and lower layer upon being inflated. Instead, the inflated ball will become a final shape as shown in FIG. 3, which is flattened to some extent as compared with a really spherical shape.
FIG. 4 shows yet another type of inflatable ball also comprising three layers, namely an upper layer 41, an intermediate layer 42 and a lower layer 43 of thermoplastic sheets, among which upper layer 41 and lower layer 43 are formed by material having smaller elastic modulus as compared with intermediate layer 42. As can be seen in FIG. 4, though the final shape of such a ball in an inflated state is nearer to a real sphere, as compared with FIG. 3, due to its weaker restoring (resisting) force against inflation, part of the material in the central region B of upper layer 41 tends to move radially outward toward its peripheral region A upon inflation. This material movement makes the thickness of the upper layer 41 non-homogeneous, thinner in central region B and thicker in peripheral region A similar situation occurs in the lower layer 43. Thus, in an inflated state, the inflatable ball is apt to break in the central region B of upper layer 41 or lower layer 43, and words or patterns printed on the outer surface of the ball will be greatly distorted in the same region B.